Peak strength and its failure criteria of anisotropic rocks have been studied and reported. However, few publications referred to residual strength behavior of anisotropic rocks. A laboratory testing technique, called the multiple failure state of triaxial test, was adopted to evaluate the residual strength behavior of anisotropic rocks in triaxial compression. A series of tests was performed on core samples of argillite. The experimental results show the fact that the triaxial residual strength is directly related to the inclination angle of specimen foliation with respecto the axial loading direction. The minimum triaxial residual strength occurs in the specimen with 30 ø of inclination angle; and specimens with 0 ø or 90 ø inclination have maximum triaxial residual strength. Analyzing the experimental data by the basis of the Mohr-Coulomb failure criterion, the residual cohesive strength strongly depends on the inclination of foliation. Nevertheless, the residual friction angle is independent of the inclination of foliation. A new failure criterion of triaxial residual strength for anisotropic rocks, modified fi:om the Hock and Brown failure criterion, is proposed in this paper. The criterion correlates closely with the experimental data of argillite specimens and can be extended to estimate the triaxial residual strength for inherent anisotropic rocks with discontinuities.

Many rocks are anisotropic, due either to the processes that form rocks, or to later processes that move and deform them. The mechanical properties of anisotropic rocks vary with direction. This variation often relates to the existence of well-defined rock fabric elements in the form of bedding, stratification, layering and foliation. In most practical cases, anisotropic rocks can be modeled either as orthotropic or transversely isotropic materials. Orthotropy implies that three orthogonal planes of elastic symmetry exist at any point in the rock, and that these planes have the same orientation throughout the rock. Transverse isotropy behaves that, at any point in the rock, there is an axis of symmetry of rotation and that the rock has isotropic properties in a plane normal to this axis. Transverse isotropy is generally characteristic of foliated metamorphic rocks (e.g. argillite, slate, phyllite and gneiss), stratified sedimentary rocks (e.g. shale and sandstone), and rocks cut by a set of regular joints. The directional response of strength and deformation on the rocks acting by external loads is called by strength and deformability anisotropy, respectively. Both anisotropic properties are common for stratified sedimentary, foliated metamorphic and regularly jointed rocks. To evaluate the stability of engineering structures built in/on these types of rocks, anisotmpy has to be considered (Liao & Wang 1998).

This paper aims to investigate the triaxial residual strength behavior of a transversely isotropic rock. The details related to anisotropy in deformation, tensile strength, and stress and strain analysis, can refer to the literature (Liao et, al. 1997). Generally, the value of failure strength for geo-materials is between' that of peak and residual strength (Adachi & Takase 1981). Depends on the types of engineering problems, either one of the two strengths can be selected for engineering analysis. Many authors have investigated in detail the behavior of triaxial peak strength of anisotropic rocks (Jaeger 1960, McLamore & Gray 1967, Donath 1972, Nova 1980). They concluded that the triaxial peak strength of anisotropic rocks (transversely isotropic rocks) varies according to the inclination of discontinuity in specimens.

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